Scientists in the United States studying the growth of human prostate cells in mice found that feeding the animals the chemical bisphenol A (BPA) almost tripled their risk of cancer or pre-cancerous changes.

The doses of BPA given to the mice were relatively the same as those commonly seen in pregnant women.

BPA is widely used to soften plastics, but there have been serious concerns about its ability to mimic the hormone estrogen. The chemical is now banned from babies’ feeding bottles in Canada, the UK and the rest of the European Union.

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Professor Gail Prins, from the University of Illinois at Chicago, who led the new research, said BPA was “very hard to avoid” despite the fact that it has been linked to several types of cancer in laboratory animals.

“Our research provides the first direct evidence that exposure to BPA during development, at the levels we see in our day-to-day lives, increases the risk for prostate cancer in human prostate tissue.

“The findings of adverse effects of BPA in human tissue are highly relevant and should encourage agencies like the Food and Drug Administration (FDA) to re-evaluate their policies in the near future.”

The study, published online in the journal Endocrinology, involved implanting human prostate stem cells taken from deceased young adult men into male mice.

Prostate stem cells – immature “incomplete” cells – arise during early fetal development and maintain a man’s prostate tissue throughout his life.

To mimic exposure to BPA during embryonic development, the researchers fed the mice the chemical for two weeks as the implanted stem cells in their bodies transformed into adult prostate tissue.

“The amount of BPA we fed the mice was equivalent to levels ingested by the average person,” said Prof Prins. “We didn’t feed them exorbitantly high doses.”

After the tissue had been allowed to mature for one month, the mice were treated with estrogen to mimic naturally rising levels of the hormone seen in aging men. Increasing levels of estrogen in men later in life is one of the known drivers of prostate cancer.

Tissue was collected after two to four months and analyzed for signs of prostate cancer.

The scientists found that a third of the samples contained either pre-cancerous changes, or full-blown tumours, compared with just 12% of samples from a comparison control group of mice fed harmless oil instead of BPA.

If the stem cells were exposed to BPA twice, before implantation and again as they developed in the mice, they produced pre-cancerous abnormalities or cancer in 45% of the tissue samples.

Prof Prins said: ” We believe that BPA actually reprogrammes the stem cells to be more sensitive to oestrogen throughout life, leading to a life-long increased susceptibility to diseases including cancer.”